The application of liquid additives to particulate solid materials requires special processing techniques in order to achieve uniform distribution of the additives on the particulate solids. The uniform distribution of the additives is particularly important when the particulate solids include smoking materials such as tobacco and the liquid additives include flavorants.
In the processing of tobacco preparatory to the manufacture of smoking products therefrom, it is customary to apply casing or sauce materials to the tobacco in order to modify the flavor and smoking characteristics of the tobacco. Apparatus which has been conventionally used for applying the casing or sauce materials includes an elongated rotary drum or cylinder having its longitudinal axis positioned in a substantially horizontal manner but with sufficient incline to allow tobacco introduced at the higher end to move gradually through the rotating drum to the lower end where the tobacco exits. Spray nozzles positioned in the interior of the rotary drum are used to apply the casing or sauce materials to the tobacco as the tobacco moves through the drum. The inner wall of the drum is typically provided with pins, ribs or blades which impart a certain agitating action to the tobacco by causing the mass of tobacco particles to turn over as gravitational forces overcome the lifting action of the pins, ribs or blades attached to the rotating inner wall of the drum. This agitating action is not entirely satisfactory because the mass of tobacco particles tends to turn and roll down the inner wall of the drum with the net result that only the outer layer of tobacco particles in the "roll" is actually contacted with the additive spray. Thus, the additives applied to the tobacco are not uniformly distributed throughout the mass of particles and this leads to nonuniformity in the smoking qualities of smoking products prepared from the treated tobacco.
Those skilled in the art have sought to improve the uniformity of application of additives applied to tobacco by employing specially designed apparatus. One such attempt is disclosed in U.S Pat. No. 4,054,145 which describes a rather complex treating arrangement that includes a rotary winnower or jets of compressed air for propelling tobacco particles into a rotary drum. A plurality of spray nozzles are arranged to apply the desired additives to the tobacco particles as the propelled particles are descending into the lower portion of the drum. This apparatus poses operational PG,4 problems in that the speed of the rotary winnower, the pressure of the compressed air and the control of the additive spray nozzles must be carefully coordinated with respect to the feed rate of the tobacco particles to insure uniform distribution of the additives on the tobacco.
Apart from the problem of achieving uniform distribution of additives sprayed onto the tobacco particles, the rotary drums used for applying casing or sauce materials to tobacco give rise to another operational problem that can result in nonuniformity of additives in the treated tobacco. This problem is the build-up of additive and tobacco materials on surfaces within the rotary drum. Significant quantities of such materials may occasionally be dislodged from the surfaces where they accumulate and may lead to concentrated pockets of additives in the treated tobacco mass. Succeeding process steps for the treated tobacco do not completely disperse these concentrated pockets of additives. The use of wiping blades or other devices to prevent accumulation of casing and tobacco materials on surfaces of the treating apparatus are not entirely effective for that purpose since it is virtually impossible to design wiping devices that will keep all of the surface areas free of accumulated deposits.
A rotary drum design which seeks to minimize the build-up of deposits on surfaces within the drum is disclosed in West German patent publication No. 30 01 734. The rotary drum described therein is provided with pins which extend radially inwardly a short distance and which serve to lift the tobacco particles as the drum is rotated. Positioned adjacent to the inner wall at a point that coincides with the highest elevation of the inner wall during its rotational movement is an axially arranged steam pipe that is provided with a number of holes bored in the wall of the pipe. Steam is ejected from these holes and is directed upwardly against the inner wall of the drum to remove any tobacco particles which may adhere to the inner wall. A number of flexible scrapers attached to the inner wall in the staggered, helical pattern provide a wiping action on the upper surface of the steam pipe to prevent build-up of deposits on the pipe and to prevent the holes in the steam pipe from becoming clogged. A wiping action is not, however, applied to a second pipe and associated nozzles through which the casing or sauce materials are directed. Also, the flexible wiping blades themselves present structures which are conducive to the accumulation of deposits on the inner wall of the drum. Additionally, the numerous lifting pins installed on the inner wall represent yet further structural elements around which tobacco and additive materials accumulate.